Using Carbon and Hydrogen Isotopes to Quantify Gas Maturity,Formation Temperature,and Formation Age — Specific Applications for Gas Fields from the Tarim Basin,China

The kinetic fractionation model for hydrogen isotope fractionation for methane, ethane and propane formation is tested in this study. The model agrees very well with the current existing model of carbon isotope fractionation for coal-derived gas from the Kuqa depression, Tarim basin, China. The strong correlation of carbon and hydrogen isotopes between theory and field data proves that it is unlikely that hydrogen isotopes will exchange with water under the gas formation condition. Using both gas carbon and hydrogen isotopes can further constrain our prediction of gas maturity, formation age and accumulation patterns for a natural gas system. Based on the carbon and hydrogen isotope fractionation model and field data, our results show the gas in the Kuqa depression was overmature in the central depression with R_o values up to 1.9–2.0% in the Kela 2 gas field and the gas maturity was much lower in the southern Front Uplift with R_o values ranging from 1.3% to 1.7%, which agree well with the distribution characteristics of the maturity of the local source rocks. However, the predicted gas maturity in the Front Uplift was relatively higher than that of the local source rocks, which probably indicates natural gases in the Front Uplift were migrated from the central depression. Our prediction demonstrates that natural gases in the Kuqa depression were formed during the last 3–5 million years and the gas formation temperature was 170–200°C, which is consistent with the burial history of the depression. According to our results, the potential accumulation pattern for the gas in the Kuqa depression is that gases were formed at depth and expelled from the Kuqa depression and migrated vertically along faults to some traps and formed giant gas fields, or migrated from north to south and accumulated in the Front Uplift or mixed with previous oil fields and formed condensate oil gas fields.